I. Field
The disclosed embodiments relate to wireless communications. More particularly, the disclosed embodiments relate to a novel and improved method and apparatus for detecting, at a receiver of a variable rate communication system, errors in the determination of the rate at which data has been encoded for transmission.
II. Background
FIG. 1 is an illustrative stepdiagram of a variable rate CDMA transmission system 10 described in the Telecommunications Industry Association over-the-air interface standard TIA/EIA Interim Standard 95, and its derivatives, such as, e.g., IS-95B (hereinafter referred to collectively as IS-95). This transmission system may be provided, for example, within a base station of a cellular transmission system for use in transmitting signals to mobile telephone subscriber units within a cell surrounding the base station. It may also be provided within mobile telephone subscriber units for use in transmitting signals to a base station.
A microphone 11 detects a speech signal which is then sampled and digitized by an analog to digital converter (not shown). A variable rate data source 12 receives the digitized samples of the speech signal and encodes the signal to provide packets of encoded speech of equal frame lengths. Variable rate data source 12 may, for example, convert the digitized samples of the input speech to digitized speech parameters representative of the input voice signal using Linear Predictive Coding (LPC) techniques. In an exemplary embodiment, the variable rate data source is a variable rate vocoder as described in detail in U.S. Pat. No. 5,414,796, which is assigned to the assignee of the present invention and is incorporated by reference herein. Variable rate data source 12 provides variable rate packets of data at four possible frame rates 9600 bits per second (bps), 4800 bps, 2400 bps, and 1200 bps, referred to herein as full, half, quarter, and eighth rates. Packets encoded at full rate contain 172 information bits, packets encoded at half rate contain 80 information bits, packets encoded at quarter rate contain 40 information bits, and packets encoded at eighth rate contain 16 information bits. Packet formats are shown in FIGS. 2A-2D. The packets, regardless of size, all are one frame length in duration, i.e. 20 ms. Herein, the terms “frame” and “packet” may be used interchangeably.
The packets are encoded and transmitted at different rates to compress the data contained therein based, in part, on the complexity or amount of information represented by the frame. For example, if the input voice signal includes little or no variation, perhaps because the speaker is not speaking, the information bits of the corresponding packet may be compressed and encoded at eighth rate. This compression results in a loss of resolution of the corresponding portion of the voice signal but, given that the corresponding portion of the voice signal contains little or no information, the reduction in signal resolution is not typically noticeable. Alternatively, if the corresponding input voice signal of the packet includes much information, perhaps because the speaker is actively vocalizing, the packet is encoded at full rate and the compression of the input speech is reduced to achieve better voice quality.
This compression and encoding technique is employed to limit, on the average, the amount of information being transmitted at any one time to thereby allow the overall bandwidth of the transmission system to be utilized more effectively to allow, for example, a greater number of telephone calls to be processed at any one time.
The variable rate packets generated by data source 12 are provided to packetizer 13, which selectively appends Cyclic Redundancy Check (CRC) bits and tail bits. As shown in FIG. 2A, when a frame is encoded by the variable rate data source 12 at full rate, packetizer 13 generates and appends twelve CRC bits and eight tail bits. Similarly, as shown in FIG. 2B, when a frame is encoded by the variable rate data source 12 at half rate, packetizer 13 generates and appends eight CRC bits and eight tail bits. As shown in FIG. 2C, when a frame is encoded by the variable rate data source 12 at quarter rate, packetizer 13 generates and appends eight tail bits. As shown in FIG. 2D, when a frame is encoded by the variable rate data source 12 at eighth rate, packetizer 13 generates and appends eight tail bits.
The variable rate packets from packetizer 13 are then provided to encoder 14, which encodes the bits of the variable rate packets for error detection and correction purposes. In an exemplary embodiment, encoder 14 is a rate ⅓ convolutional encoder. The convolutionally encoded symbols are then provided to a CDMA spreader 16, an implementation of which is described in detail in U.S. Pat. Nos. 5,103,459 and 4,901,307. CDMA spreader 16 maps eight encoded symbols to a 64-bit Walsh symbol and then spreads the Walsh symbols in accordance with a pseudo-random noise (PN) code.
Repetition generator 17 receives the spread packets. For packets of less than full rate, repetition generator 17 generates duplicates of the symbols in the packets to provide packets of a constant data rate. When the variable rate packet is half rate, the repetition generator 17 introduces a factor of two redundancy, i.e., each spread symbol is repeated twice within the output packet. When the variable rate packet is quarter rate, the repetition generator 17 introduces a factor of four redundancy. When the variable rate packet is eighth rate, the repetition generator 17 introduces a factor of eight redundancy.
Repetition generator 17 provides the aforementioned redundancy by dividing the spread data packet into smaller sub-packets referred to as “power control groups.” In the exemplary embodiment, each power control group comprises 6 PN spread Walsh Symbols. The constant rate frame is generated by consecutively repeating each power control group the requisite number of times to fill the frame as described above.
The spread packets are then provided to a data burst randomizer 18, which removes the redundancy from the spread packets in accordance with a pseudo-random process as described in U.S. Pat. No. 5,535,239, assigned to the assignee of the present invention. Data burst randomizer 18 selects one of the spread power control groups for transmission in accordance with a pseudo-random selection process and gates the other redundant copies of that power control group.
The packets are provided by data burst randomizer 18 to finite impulse response (FIR) filter 20, an example of which is described in U.S. Pat. No. 5,659,569, and assigned to the assignee of the present invention. The filtered signal is then provided to digital to analog converter 22 and converted to an analog signal. The analog signal is then provided to transmitter 24, which up-converts and amplifies the signal for transmission through antenna 26.
FIG. 3 illustrates pertinent components of a base station. In another embodiment, the apparatus of FIG. 3 could reside in a mobile telephone 28 or other mobile station receiving the transmitted signal. The signal is received by antenna 30, down-converted and amplified, if necessary, by receiver 32. The signal is then provided to frame rate detection unit 33, which subdivides the signal into packets and determines the corresponding frame rate for each packet. The frame rate may be determined, depending upon the implementation, by detecting the duration of individual bits of the frame. The packet and a signal identifying the detected frame rate for the packet are then forwarded to CRC unit 34 for performing cyclic redundancy checks or related error detection checks in an attempt to verify that no transmission errors or frame rate detection errors occurred. A frame rate detection error results in the packet being sampled at an incorrect rate resulting in a sequence of bits that are effectively random. A transmission error typically results in only one or two bits being in error. Usually, if a transmission error or frame rate detection error occurs, the CRC unit detects the error. “Bad” frames failing the CRC are erased or otherwise discarded by frame erasure unit 36. “Good” frames which pass the CRC are routed to variable rate decoder 40 for conversion back to digitized voice signals. The digitized voice signals are converted to analog signals by a digital to analog converter (not shown) for ultimate output through a speaker 42 of the mobile telephone.
Depending upon the implementation, no separate frame erasure unit 36 is necessarily required. Rather, CRC unit 34 may be configured merely to not output bad frames to variable rate decoder 40. However, provision of a frame erasure unit facilitates generation of frame erasure signals for forwarding back to the base station to notify the base station of the frame erasure error. The base station may use the frame erasure information to modulate the amount of power employed to transmit signals, perhaps as part of a feedback system intended to minimize transmitted power while also minimizing frame errors.
As noted above, by varying the frame rate of packets to thereby compress the information contained therein, the overall bandwidth of the system is utilized more effectively, usually without any noticeable effect on the transmitted signal. However, problems occur occasionally which have a noticeable effect. One such problem occurs if a frame subject to a frame rate detection error or a transmission error nevertheless passes the CRC. In such cases, the bad frame is not erased but is processed along with other good frames. The error may or may not be noticeable. For example, if the error is a transmission error wherein only one or two bits of encoded speech are in error, the error may have only an extremely slight and probably unnoticeable effect on the output voice signal. However, if the error is a frame rate detection error, the entire packet will thereby be processed using the incorrect frame rate causing effectively random bits to be input to the decoder, likely resulting in a noticeable artifact in the output voice signal. Noticeable artifacts caused by frame rate detection errors are such unacceptable distortions as screeches, or beeps. For some systems, it has been found that incorrect frame rate detections occur with a probability of about 0.005% yielding an incorrectly received packet and a corresponding artifact in the output voice signal about every sixteen minutes of conversation time. Although described with respect to a CDMA system using IS-95 protocols, similar problems can occur in almost any transmission system employing variable transmission rates and in related systems as well.
Due to effects of channel conditions such as noise, and multi-path fading on received parameters, rate determination algorithms (RDA) of frame rate detection units 33 do not guarantee that the received frame rate is correct. Given that this is a limitation of the RDA, it is desirable to ensure that such RDA errors don't cause audible artifacts such as screeches, or beeps. When the received frame is not suitable for accurate rate-determination due to poor channel conditions, the RDA either determines that the frame has to be erased or it assigns an incorrect rate to the packet. Typically, the speech decoder has a frame-erasure processing mechanism that perceptually smoothes the lost frames using past frames, and produces speech that is not annoying to the listener. However, if instead of a frame erasure, the RDA assigns an incorrect rate to the frame, random bits are fed into the variable rate decoder 40. Unless detected, the random bits can produce very loud, annoying artifacts such as screeches, and beeps. It is generally true that a frame-erasure does not produce as much speech quality degradations as an incorrect rate frame.
It is desirable to handle these incorrect rate frames without generating audible artifacts. It is desirable to detect an incorrect rate frame, and perform frame-erasure processing, and/or clean-up the memory states in the variable rate decoder 40, such that effects of incorrect rate determination do not propagate across many frames.
Therefore, it can be appreciated that there is a significant need for a method that detects rate determination errors in a wireless communication system, and eliminates resultant audible artifacts.